1. Field of the Invention
The present invention relates to a process for producing an alcohol by hydroformylation which comprises reacting a monoolefin with carbon monoxide and hydrogen to form an alcohol having carbon atoms greater in number by one than the carbon atoms of the monoolefin.
2. Description of the Prior Art
For last ten years, so-called oxoalcohols have been used in great amounts for a variety of purposes, for instance, as intermediates for plasticizers for various polymers, as starting materials for synthetic detergents, as intermediates for agricultural chemicals, medicines or fine chemicals such as food additives or as various solvents, and they have been constantly in demand. The manufacturers are competing one another in an effort to supply such oxoalcohols steadily and inexpensively. Under these circumstances, various new catalytic systems have been proposed and various efforts are being made for improvement of the oxo process.
Especially in view of the oil shortage in recent years, it is desired to improve the yield of alcohols from olefin raw materials and to save the energy required for the process.
In the conventional oxo processes, it is common to use a cobalt carbonyl as a catalyst. The cobalt carbonyl catalyst is highly active, whereby the reaction rate of the olefin is quite high and the selectivity of its conversion to the corresponding aldehyde is fairly high. However, the alcohol subsequently formed by hydrogenation of the aldehyde in the presence of the cobalt carbonyl catalyst is likely to react with the aldehyde in the same reaction system to form undesirable products such as acetal. Thus, such a conventional process has a drawback that the formation of the high boiling point products increases as the conversion rate of the olefin is raised. In particular, when the conversion rate of the olefin is raised to a level higher than 95 mol % under the usual industrial reaction conditions, it is almost impossible to suppress the formation of the high boiling point products. Further, the product from the oxo process will have to be hydrogenated, after removal of the cobalt carbonyl catalyst, by same means to obtain the desired alcohol. This hydrogenation is usually carried out by a method in which the product from the oxo process is, after removal of the catalyst, passed through a fixed bed catalyst column under hydrogen pressure at a high temperature. As the fixed bed catalyst, a copper chromite catalyst, or a nickel or cobalt catalyst supported on silica or alumina is usually used. Depending upon its surface conditions, such a catalyst tends to lead to various side reactions to produce high boiling point products. Thus, various difficulties are involved in the selection of the hydrogenation catalyst.
In an attempt to overcome the difficulties, a cobalt organophosphine complex catalyst having a biphilic ligand such as trivalent phosphorus has been developed whereby the hydroformylation of the olefin and the hydrogenation of the formed aldehyde are conducted in a single step, as disclosed in Japanese Examined Patent Publication No. 1402/1964. According to this process, the formation of the high boiling point products can substantially be suppressed and the stability of the catalyst is improved whereby the pressure can be reduced. A further feature of this process is that when a straight chained monoolefin is used as the starting material, the selectivity of the conversion of the formed aldehyde to the straight chained alcohol is quite high as compared with the other conventional processes. However, such a catalyst having a biphilic ligand composed of Co and trivalent phosphorus has an increased hydridic character and therefore naturally tends to lead to direct hydrogenation of the olefin to the corresponding paraffin. Accordingly, even when the optimum conditions are selected and a straight chained terminal olefin is used as the starting material, a considerable amount of the paraffin will be formed as an impurity although it depends upon the structure of the trivalent phosphorus. When a branched olefin or an inner olefin is used, the conversion of the olefin into the paraffin by the direct hydrogenation takes place at a selectivity of at least 20 mol % of the converted olefin.